This invention relates to a modular conveyer chain, and more particularly to an improved chain link for use in construction of a modular conveyer chain.
Manufacturing and production facilities utilize modular conveyer chains to transport products or articles of production from one location to another. Conventional modular conveyor chains are typically comprised of multiple thermoplastic chain links or modules. The links making up the modular conveyer chain typically have a plurality of spaced link ends which intermesh with complementary spaced link ends projecting from a link or links in an adjacent row. The individual chain links are usually similar in width and may be arranged in a bricked configuration. The intermeshing link ends are joined or hinged together by a connecting pin that permits the adjacent chain links to pivot with respect to each other.
The chain links are typically joined together to form an endless conveyor chain that is usually driven by a drive sprocket. The modular conveyor chains are subjected to tensile forces that tend to separate the individual chain links when the chain is placed under a load.
Conventional chain links are typically made of thermoplastic (e.g. acetal, polyester, nylon and polypropylene). The choice of the polymer used for the chain link usually depends on the physical properties which are desired (i.e. high tensile strength, high fatigue strength, low friction, chemical resistance and/or suitability for use under extreme cyclic temperatures) in the chain link. The tensile strength and fatigue strength of the chain link are especially important because a chain link having these increased mechanical properties increases the overall tensile strength of the modular conveyor chain and reduces chain stretch due to loading.
Modular conveyer chains are often used to carry goods from one location to another location where the temperature of the environment at the two locations is significantly different. The individual chain links expand as the temperature of the chain increases, and contract as the temperature of the chain decreases. As the individual chain links expand or contract, the overall length of the conveyer chain varies significantly as a result of a high coefficient of thermal expansion that is commonly associated with most thermoplastics.
A typical application where a modular conveyor belt is subject to extreme cyclic temperatures is in a conveyor chain used to transport cans or bottles through pasteurizers in breweries. The high temperatures in a pasteurizer combined with the slow movement of the cans or bottles through the pasteurizer when the chain is under a tensile load may cause the chain to stretch such that the bottom canteary section of an endless conveyer chain sags. This chain stretching may also effect the performance of the interaction between the drive sprocket and chain links. In addition, in double deck conveyor systems, the sagging can become so great that the bottom canteary section of the top conveyor of an endless conveyer chain interferes with bottles located on a lower conveyor chain.
One known method for increasing the tensile strength and the fatigue strength of the overall modular conveyor chain is to use metal links in combination with the thermoplastic chain links. The combination of thermoplastic links and metal links causes the loads on the modular conveyer to be carried primarily by the metal links. One of the problems associated with combining links made from two different materials to form a modular conveyor is that there are significant bending stresses generated within the thermoplastic chain links due to the differences in the modulus of elasticity, coefficient of friction and coefficient of thermal expansion between the thermoplastic chain links and the metal chain links.
Plastics manufacturers have increased the tensile strength of thermoplastics by adding filler to the polymer as the raw polymer is being manufactured. The filler is typically in the form of long fibers. Manufacturers of long fiber reinforced thermoplastics, such as Ticona and DuPont, provide technical literature to their customers which indicates that increasing the amount of filler within the raw polymer increases the tensile strength of the molded polymer. The technical literature also provides results for tensile tests performed on different thermoplastics where the percentage of filler within the polymer varies. The tests were performed in accordance with ASTM standards and indicate that the tensile strength of the thermoplastics increases as the weight percent of filler within the raw polymer increases. The technical literature shows test results for polymers that include up to 60 weight percent filler within the polymer.
The present invention is an improved chain link for use in constructing a modular conveyor chain. The chain link includes a plurality of spaced link ends that extend from the body of the chain link. The link ends are adapted to intermesh with complementary spaced link ends projecting from a link or links in an adjacent row. The link ends include openings which are axially aligned and adapted to receive a connecting pin that runs through the openings to pivotally connect the link with an adjacent chain link or links. The chain link is molded from a thermoplastic material that includes a filler, preferably glass fiber, which improves the mechanical properties of the chain link. The amount of filler within the molded thermoplastic material should maximize the fatigue strength and tensile strength of the molded chain link in environments where the temperature can vary significantly. The chain link comprises less than about 30 weight percent of filler based on the weight of the molded chain link; and preferably between the range of about 5 to 25 weight percent; and more preferably between the range of about 10 to 20 weight percent. The filler is preferably in the form of long strands which have a length between 0.125 inches and 0.5 inches.
The modular chain link is preferably injection molded from a strong base polymer in order to provide ample strength and corrosion resistance to the chain link. In addition, the links are preferably molded in a die having a relatively high temperature because during molding a layer consisting of unfilled polymer forms near the surface of the link and increasing the temperature of the die causes the layer to be thicker. Positioning the fibers as far as possible from the surface of the link is crucial because the fibers can be very abrasive and during operation of the modular conveyor chain there is commonly point contact between the connecting pins and the internal edges of the link ends. The point contact results in a significant amount of relative motion between the connecting pins and link ends. This type of motion can cause extreme wear, especially when the abrasive fibers are near the outer surface of the links. Increased wear reduces the operating life of the modular conveyor chain.
During conventional injection molding as hot liquid elastomer material flows around an obstruction in the mold (e.g., a core pin), two flow fronts having a partially solidified skin surface meet. Where the skin surfaces meet a less homogenous blend of polymer is formed. The areas where the surfaces meet are conventionally referred to as weld lines or flow lines. The mechanical properties of the molded chain link at these weld lines are significantly degraded, especially the tensile strength, stiffness, fatigue strength and impact resistance.
The orientation of the fibers within the molded chain link can be manipulated by locating the gates which supply liquid polymer into the molding die in a particular configuration. The gates on the molding die are preferably located such that the fibers are oriented within the chain link in substantially the same direction as the direction of the travel of the modular conveyor chain. Orienting the fibers within the modular chain link in the direction of chain travel significantly increases the tensile strength and the fatigue strength of a modular conveyor chain that is assembled from the individual chain links.
An object of this invention is to provide a chain link for use in constructing a modular conveyor chain that has a low friction surface, high tensile strength, high fatigue strength, minimal coefficient of thermal expansion and a more stable modulus of elasticity as the operating temperature increases. Increasing the strength of thermoplastic modular conveyor chains is critical because many conveyor applications require a high strength conveyor chain.
Another object of this invention is to provide a chain link for use in constructing a modular conveyor chain that has increased fatigue strength and tensile strength over a range of operating temperatures. Increasing the useful range of operating temperatures where a modular conveyor chain can function effectively allows the chain to be used in a greater number of applications.
A further object of this invention is to provide a chain link for use in constructing a modular conveyor chain that is more effective in the types of applications where modular conveyor chains are typically used. Modular conveyor chains are typically used in pasteurizers, bottle and can warmers, industrial microwave ovens, shrink wrap tunnels and freezers.
Yet another object of the invention is to provide a modular conveyor chain that will resist stretching due to mechanical loading in a variety of environmental conditions including high temperatures and corrosive environments.
Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims and drawings.